The invention relates to a system and method for controlling an internal combustion engine coupled to an emission control device. More particularly, the invention relates to a system and method for controlling the internal combustion engine in response to a corrected NOx sensor output.
Internal combustion engines are coupled to an emission control device known as a three-way catalytic converter designed to reduce combustion by-products such as carbon monoxide (CO), hydrocarbon (HC) and oxides of nitrogen (NOx). Engines can operate at air-fuel mixture ratios lean of stoichiometry, thus improving fuel economy. However, the amount of NOx released during lean operation can be greater than that at rich operation or at stoichiometry, which compromises emission control in the vehicle. To reduce the amount of NOx released during lean operation, an emission control device known as a NOx trap, which is a 3-way catalyst optimized for NOx control, is usually coupled downstream of the three way catalytic converter. The NOx trap stores NOx when the engine operates lean. After the NOx trap is filled, stored NOx needs to be reduced and purged. In order to accomplish this, engine operation is switched from lean to rich or stoichiometric, i.e., the ratio of fuel to air is increased.
One method of determining when to end lean operation and to regenerate a NOx trap by operating the engine rich or near stoichiometry is described in EP 0,814,248. In particular, a sensor capable of measuring the amount of NOx in exhaust gas exiting from the NOx trap is installed downstream of the trap. The operation condition of the engine is switched from lean to stoichiometric (xe2x80x9cstoicxe2x80x9d) or rich when the output value of the NOx sensor is greater than or equal to some predetermined value. This causes the nitrogen oxide absorbed in the NOx trap to be decomposed and discharged, and allows the engine to be operated under lean conditions again.
The inventors herein have recognized a disadvantage with the above approach. In particular, with certain No. sensors, when a NOx purge is performed, a small amount of reducing agent (for example, hydrocarbon or carbon monoxide) escapes through the NOx trap and is absorbed by the NOx sensor, thus saturating it. This can cause the sensor to give an erroneously high or low reading. This reading can cause over- or under-estimation of the tail-pipe NOx, and therefore may cause unnecessary NOx purges, which can degrade fuel economy. Also, it may cause incorrect estimation of NOx in grams per mile and degrade vehicle emission strategy operation.
An object of the present invention is to provide a method for determining the correct amount of tail-pipe NOx emissions for a certain time period after a NOx purge, and for adjusting an engine control strategy in response to corrected NOx sensor output.
The above object is achieved and disadvantages of prior approaches overcome by a method for controlling an internal combustion engine coupled to an emission control device, the engine coupled to an exhaust sensor providing first and second signals respectively indicative of first and second quantities. The method includes the steps of determining when the second signal deviates from the second quantity based on the first signal; adjusting the second signal in response to said determining step; and adjusting an engine operating parameter based on the adjusted second signal.
An advantage of the above aspect of the invention is that a more precise method for calculating tailpipe NOx emissions is achieved, which improves fuel economy. By adjusting the NOx sensor reading during the period of reductant deposit on the sensor, it is possible to eliminate the effects of such deposit on the sensor. In other words, the more precise measurement of NOx makes it possible to eliminate unnecessary NOx purges, thus allowing the engine more lean running time, and improving fuel economy. Also, knowing a more accurate amount of NOx emissions allows for improved emission control strategy. It is an especially advantageous aspect of the present invention that a first output of the sensor can be used to determine when a second output of the sensor deviates from the parameter to be measured.
In another aspect of the present invention, the above object is achieved and disadvantages of prior approaches overcome by a method for controlling an internal combustion engine coupled to an emission control device, the engine coupled to an exhaust sensor providing a first signal and a second signal respectively indicative of an exhaust gas air-fuel ratio and a NOx level, the method including the steps of: determining the NOx level based on a first engine operating parameter when the first signal indicates the exhaust air-fuel ratio is richer than a first predetermined value;, determining the NOx level based on the second signal when the first signal indicates the exhaust air-fuel ratio is leaner than a second predetermined value and reductant deposited on the sensor is depleted by excess oxygen in the lean exhaust gas; and adjusting a second engine operating parameter based on the determined NOx level.
By using the actual NOx sensor reading in regions where it is indicative of actual NOx, an accurate control system is obtained. Further, it is possible to determine when the NOx sensor reading deviates from the actual NOx level by monitoring the amount of oxygen in the exhaust gas. Therefore, when such deviation occurs, it is possible to make corrections to the NOx sensor reading. Also, it is possible to determine when the sensor starts reading correctly by determining when the reductant is oxidized by lean exhaust gas.
Other objects, features and advantages of the present invention will be readily appreciated by the reader of this specification.